Still from American Sniper. The comic is never named, but if you look closely, you can see that it's issue No. 1 of the sixth volume of Punisher, released in January of 2004 (making it very believable as something Biggles could be reading during the platoon's 2004 deployment).
A Dr. Garry Choy, MD, Diagnostic Radiology, answered. Radiation fibrosis is the result of prior radiation exposure that results in scarring of an organ such as breasts, lungs, etc. Post radiation treatment fibrosis is the typical cause.
Radon and its decay products release radioactive energy that can damage lung tissue in a way that causes the beginning of lung cancer. The more radon you are exposed to, and the longer the exposure, the greater the risk of eventually developing lung cancer. Radon is the second leading cause of lung cancer in the United States, resulting in 15,000 to 22,000 deaths per year.